In the drilling of earth boreholes, especially in the drilling of shallow water wells in unconsolidated formations such as sand and cobblestones, it is fairly common to encounter water flowing out of the well to the earth's surfaces which flow must be controlled. To merely pour wet cement into the earth borehole will quite often result only in the flowing water pushing the wet cement out of the borehole, with little or no reduction in the water flow.
An associated problem involves the fracturing of the formation while drilling, resulting in the fluid in the borehole being lost to the formation, commonly referred to as "lost circulation".
Still another problem resides in the grouting of tubulars in an earth borehole. Grouting is the placement of a suitable material with a low permeability and high structural stability into the annular space between the casing and the wellbore, or between the casing and the inner pipe such as tubing. The main functions of grouting and sealing a well or earthen cavity are to (1) restore the earth formation outside the casing to its original condition, (2) prevent commingling of two aquifers, (3) stop seepage of polluted surface water downwardly along the exterior of the casing into the well, and (4) preserve the hydraulic characteristics of an artesian formation and to prevent leakage upwardly along the exterior of the casing.
Two types of material are frequently used for grouting a well. One is bentonitic grout consisting of a high-yield, swellable bentonite clay, particularly sodium-based bentonite containing sodium montmorillonite; and another is cementitious grout consisting of cement or silicate-based material with the addition of a small percentage by weight (4% to 8%) of sodium bentonite to enhance grout bonding strength. Each type of grouting material has its advantages and disadvantages. Cementitious grouts are, in general, less economical than bentonitic grouts, and have a higher slurry density (13 to 15 lb/gal) and a higher slurry pH (above 12.5) than that of bentonitic grouts. The major drawbacks of cementitious grouts are that they are not cost effective, and they have a tendency to fracture the surrounding earth formations due to their heavy slurry weight (density). U.S. Pat. Nos. 4,463,808, 4,948,428, and 5,389,146 each disclose the use of bentonitic grouts.
U.S. Pat. No. 4,463,929 to Mason et al. describes a method of preparing a bentonitic grout for sealing an earthen borehole. It involves a two-step preparation by adding a granular, water swellable clay such as Wyoming sodium bentonite to a pre-mixed water solution containing 0.1% to about 0.5% by weight of a water dispensable hydrolyzed polyacrylamide polymer. The amount of swellable clay should be in a concentration from 2 to 4 pounds per gallon of pre-mixed polymer dispersed in water. The polymer in this patent is in emulsion form and is dispersed in water first before it comes in contact with clay particles.
U.S. Pat. Nos. 4,948,428 to Liao describes a pumpable, rehydratable grout composition comprising a water-swellable clay, such as sodium bentonite, and an aqueous mixture comprised of a hectorite, non-swellable clay, an alkaline metals tetraborate, an alkaline metal carbonate and an alkaline metal chloride, the grout composition being useful in the sealing of earthen formations such as the grouting of conduits in wellbores. The disclosure involves a two-step grout preparation. The first step is to add a fixed amount of the aforementioned aqueous mixture to make-up water, and the second step involves the addition of water-swellable sodium bentonite clay to the above aqueous, inorganic solution. The aqueous mixture contains a mixture of inorganic alkaline salts intended for use as a dispersant to disperse clay particles so that they will not stick together while swelling once in contact with water. The principle mechanism of this patent is dispersion initiated by the dispersants in the aqueous mixture.
U.S. Pat. No. 5,389,146 to Liao discloses a grouting composition, containing no organic polymeric substance, mixed with freshwater in one-step addition to provide a pumpable, rehydratable, and easily applied grouting slurry. This patent describes a grout composition comprised of a mixture of water-swellable clay, such as granular sodium bentonite (88 to 90%) with a particle size distribution ranging from 150 to 600 micrometers, sepiolite (1 to 8%) and a clay stabilizer (4 to 8%). The composition can be added to freshwater directly to provide a pumpable grout slurry for sealing the annular space around a well casing and plugging abandoned wells. The principle of this patent relies on the clay stabilizer in the grout composition as a dispersant to disperse sodium bentonite clay particles so that more clay particles can be introduced into the slurry to form a high solids grout.
U.S. Pat. No. 4,836,940 to William Alexander discloses the use of a homogeneous pellet made from a mixture of sodium bentonite with a water absorbing polymer to provide sealing and plugging of earth formations surrounding an earth borehole. By way of further example, in U.S. Pat. No. 4,551,256 to Kita et al., there is a disclosure of composite inorganic powder particles coated with various water absorbing resins. The composite particles are pulverized to result in powder particles which are then used to make a slurry for drilling earth boreholes.
U.S. Pat. No. 2,836,555 to Armentrout discloses a pellet containing compressed, dehydrated, swellable bentonite clay coated with a water insoluble coating which is impervious to the ingress of water except at a restricted entrance, whereby the pellet may be introduced into a water containing mixture, pumped into a well, and ingress of water to the clay within the coating is controlled to an extent which will cause the clay to expand and rupture the pellet, an event which is delayed until the pellet has had an adequate opportunity to be carried by the mixture to the desired location, after which the clay may absorb water, swell, and rupture the pellet, exposing more of the clay to water absorption and swelling. The coating used in this patent contains a water-insoluble compound which forms an impermeable film on the surface of the pellet except in a small restricted area. When this coated pellet is introduced in a borehole, the hole on the surface of the pellet is the only spot where water comes in contact with the swellable clay. The surface coating used on the pellet is preferably of a thermosetting character so that it will not soften or become plastic under the temperature conditions encountered in deep wells. The particular nature of the water insoluble coating may vary considerably. Urea formaldehyde adhesives are suitable for this purpose in that they are thermosetting and are water resistant. Other synthetic resins which are water resistant and which will not soften under the temperatures encountered in wells may be used in lieu thereof. Such resins frequently are soluble in volatile organic solvents such as a mixture of ethyl alcohol (1%) and methylene chloride (9%), and a mixture of butyl acetate, toluene, ethyl alcohol and butanol, and may be applied in the form of solutions after which the solvents evaporate away, leaving the resins enclosing the pellet and forming a water resistant and temperature resistant coating. The preferable coating described in the patent includes cellulose acetate, urea formaldehyde, uric resins, polystyrene and nitrocellulose. The coatings aforementioned prevent ingress of water to the bentonite clay except through a small entrance. This entrance may be formed by puncturing or drilling a hole into the body of the coated pellet. The entrance and the hole are preferably of a very small size, so small that while water may be forced therein under pressure, water will not readily enter, particularly against any air that may be entrapped in the hole.
Recovering Lost Circulation Materials
During the drilling and production of wells, such as oil, gas or water wells, various problems sometimes occur which, if not corrected, result in the loss of considerable well production and even the loss of the well itself. These problems include blowouts, lost circulation and channeling. Surface blowouts occur when the drill bit cuts into a high pressure zone of gas, oil or water which can blowout the drilling mud and sometimes even the drill stem, bit and surface equipment resulting in their loss and destruction also. Underground blowouts occur when a fluid in one porous formation flows into the wellbore and out of the wellbore into another porous formation. Lost circulation of drilling mud occurs when the drill bit cuts through a porous formation at a pressure such that drilling mud coming down the drill stem and well casing is lost to the formation having a lower internal pressure. Channeling occurs when a fluid in one porous formation flows through continuous passages in cement behind well casing into another porous formation at a different depth in the well.
It is well known in the prior art that a variety of different substances are pumped down wellbores in attempts to reduce the large losses of drilling fluid to fractures and the like in the surrounding formation. Different forms of cellulose are the preferred materials employed. Some other substances which have been pumped into wellbores to control lost circulation are almond hulls, walnut hulls, bagasse, dried tumbleweed, paper, coarse and fine ica, and even pieces of rubber tires. These and other prior art additives are described in U.S. Pat. No. 4,498,995.
Another process that is employed in the prior art to close off large lost circulation problems is referred to in the art as gunk squeeze. In the gunk squeeze process, a quantity of powdered bentonite is mixed in diesel oil and pumped down the wellbore. Water injection follows the bentonite and diesel oil. If mixed well, the water and bentonite will harden to form a gunky semi-solid mass, which will reduce lost circulation. Problems frequently occur in trying to adequately mix the bentonite and the water in the well. The bentonite must also be kept dry until it reaches the desired point in the well. The method is disclosed in U.S. Pat. No. 3,082,823.
U.S. Pat. No. 4,261,422 describes the use of an expandable clay such as bentonite or montmorillonite which is dispersed in a liquid hydrocarbon for injection into the well. After injection, the bentonite or montmorillonite will expand upon contact with water in the formation. Thus, it is hoped that the expanding clay will close off producing intervals but not harm oil producing intervals.
It has also been proposed to mix bentonite with water in the presence of a water soluble polymer which will flocculate and congeal the clay to form a much stronger and stiffer cement-like plug than will form if only bentonite is mixed with water. U.S. Pat. No. 3,909,421 discloses such a fluid made by blending a dry powdered polyacrylamide with bentonite followed by mixing the powder blend with water. U.S. Pat. No. 4,128,528 discloses a powdered bentonite/polyacrylamide thickening composition prepared by mixing a water-in-oil emulsion with bentonite to form a powdered composition which rapidly becomes a viscous stiff material when mixed with water. U.S. Pat. Nos. 4,503,170; 4,475,594; 4,445,576; 4,442,241; and 4,391,925 each disclose the use of a water expandable clay dispersed in the oily phase of water-in-oil emulsion containing a surfactant to stabilize the emulsion and a polymer dispersed in the aqueous phase. When the emulsion is sheared, it breaks and a bentonite paste is formed which hardens into a cement-like plug. These patents disclose the use of such polymers as polyacrylamide, polyethylene oxide and copolymers of acrylamide and acrylic or methacrylic acid.
An encapsulated material which has been proposed for use in boreholes is described in U.S. Pat. No. 4,078,612. The patent describes an explodable material encapsulated in natural gums slurried in a liquid vehicle. The material is pumped into the formation around the wellbore and exploded to decrease the permeability of the formation.
All of the above-discussed prior art suffer from a common problem, that of being able to offer a one-step solution to the problem of being able to take a product to the field, mix it with water, and pump the mixture into the earth borehole as a grouting solution, as a fracture sealing solution or as a plugging solution.